The state of the art in oriented crystalline film manufacturing is known as epitaxial growth. The epitaxial growth of single crystal films of inorganic materials on inorganic substrates is widely used in modern semiconductor technology. There are two basically different processes: gas- or vapor-phase epitaxy (VPE), where thin layers are deposited onto substrates from gas or vapor mixtures, and liquid-phase epitaxy (LPE), where the growth proceeds from liquid solutions or melts. It must be noted that epitaxial growth requires using crystalline substrate with parameters of the crystal unit cell matched with that of the growing thin crystal film. In this case, the crystal structure repeats that of the substrate and the anisotropy of the physical properties of such films is determined by the type of the crystal lattice of the semiconductor material employed [see: Recent Developments in the Theory of Epitaxy, J. H. van der Merwe, in Chemistry and Physics of Solid Surfaces V, Eds. R. Vanselow and R. Howe, Springer-Verlag, N.Y. (1984), p. 365–401, and Growth from the Vapor Phase, in Modern Theory of Crystal Growth I, Ed. A. A. Chemov, Springer-Verlag, N.Y. 1983, Ch. 9].
There is a known method for the epitaxial growth of thin layers composed of large anisotropic organic molecules on inorganic substrates. According to this, the deposition process or mass transfer is produced via a VPE process in a vacuum chamber. This VPE technique was used to obtain the layers of organic molecules on graphite, alkali halide, and some other materials. [see: N. Uyeda, T. Kobayashi, E. Suito, Y. Harada and M. Watanabe, J. Appl. Phys. 43(12), 5181 (1972); M. Ashida, Bull. Chem. Soc. Jpn. 39(12), 2625–2631, 2632–2638 (1966); H. Saijo, T. Kobayashi and N. Uyeda, J. Crystal Growth 40 118–124 (1977); M. Ashida, N. Uyeda and E. Suito, J. of Crystal Growth 8, 45–56 (1971); Y. Murata, J. R. Fryer and T. Baird, J. Microsc., 108(3), 261–275 (1976); J. R. Fryer, Acta Cryst. A35, 327–332 (1979); M. Ashida, N. Uyeda and E. Suito, Bull. Chem. Soc. Jpn. 39(12), 2616–2624 (1966); Y. Saito and M. Shiojiri, J. Crystal Growth 67, 91 (1984); and Y. Saito, Appl. Surf. Sci. 22/23, 574–581 (1985)].
Also known are the methods for epitaxial growth and polymerization of synthetic polymers and biopolymers on alkali metals halide substrates from solutions, melts, and vapor phase. There are examples of using other inorganic minerals as substrates [see. A. McPherson and P. J. Schlichto, J. Cryst. Growth 85, 206 (1988)].
There are several disadvantages inherent in inorganic single crystals, which limit the possibilities of using such crystals as substrates for epitaxial growth. In particular, the number of single crystal materials suited for epitaxial growth is rather restricted because the crystal surface can be reactive, and/or covered with oxides, and/or contain adsorbed water molecules. The substrate can be nontransparent, possess undesired electronic and/or thermal properties, and so on. The major restriction is based on the requirement of coinciding or co-dimensioning crystal lattices of substrate and growing crystal film. Another restriction is the size of single crystal substrate that is available for reasonable cost. Most of single crystals are produced in limited sizes and at high expense.
There is a known method for the molecular beam epitaxial growth of organic thin films [A. Korna, Molecular Beam Epitaxial Growth of Organic Thin Films, Prog. Crystal Growth and Charact., Vol. 30, pp. 129–152, 1995] and the formation of layered films involving a substrate, at least one surface of which is covered, at least partly, with the first layer (called “seed” layer, which will be referred to below as the alignment layer) of a crystalline, uniaxial oriented organic compound, and contains the second layer of a crystalline uniaxial oriented organic compound formed above the first layer, whereby the second layer is subjected during its growth to the aligning action of the first layer. For brevity, the second layer will be referred to below as the epitaxial layer.
The aforementioned known method was intended for the obtaining of layers of organic compounds consisting of a considerable extent of planar molecules composed of chains and (which is more favorable) rings. In a side view, such molecules appear as short straight segments, while viewed from top they appear as circles or ellipses (if the molecular plane is inclined relative to the viewing direction). As a rule, such molecules are packed into stacks, and the stacks form a crystal structure. The molecular stacks are characterized by strongly developed π bonds (pi bonds). From the standpoint of crystallography, such crystals represent uniaxial crystals and are characterized by the b-axis coinciding with the stack axis. Orientation of the crystal will be characterized by the angle between the b-axis and the normal to the substrate surface.
Organic compounds preferred for the obtaining of multilayer films by said known method represent polycyclic aromatic hydrocarbons and heterocyclic compounds. Polycyclic aromatic hydrocarbons are described in literature [see: Morrison and Boyd, Organic Chemistry, Third Edition, Allyn and Bacon Inc., Boston, (1974), Chapter 30; for heterocyclic compounds, see: Ibid, Ch. 31]. Among polycyclic aromatic hydrocarbons, of most interest from the standpoint of the film growth method under consideration are naphthalenes, perylenes, anthracenes, coronenes, and related derivatives. Among heterocyclic compounds (with S, N, and O heteroatoms), the most attractive are phthalocyanines, porphyrins, carbazoles, urines, pterins and their derivatives.
According to said known method, when an epitaxial layer of an organic compound is grown by VPE above an alignment layer, the crystal structure of this epitaxial layer is determined by that of the alignment layer; since the second layer is grown epitaxially on the first one, the b-axis direction in the epitaxial layer will also depend on that in the alignment layer.
Despite all advantages of said known method, it is not free of significant drawbacks. In particular, before growing an organic epitaxial layer possessing a desired orientation of planar organic molecules and required crystallographic parameters, in the general case, it is necessary to apply an alignment layer onto the substrate, which is an independent difficult task. It should be noted that, in said known method, the physical (crystallographic) properties of the alignment layer and the orientation of molecules in this layer significantly depend on the substrate temperature during the growth of this layer. This circumstance may also present a certain disadvantage. Note that any organic compound is characterized by a definite permissible temperature range, which requires special elaboration of the growth technology in application to each particular compound.
According to said known method, a constant temperature regime and vacuum level have to be maintained in the chamber during the whole epitaxial growth process. Any breakdowns in the temperature and vacuum regime lead to the appearance of defects in the growing layer, whereby both crystallographic parameters and the orientation of molecular layer exhibit changes. This sensitivity of the process with respect to instability of the technological parameters can be also considered as a shortcoming of said known method, which is especially pronounced during the formation of thick (1 to 10 μm) epitaxial layers.
Another disadvantage of said known method is the need in sophisticated technological equipment. The reactor chamber must hold ultrahigh vacuum (down to 10−10 Torr) and must withstand considerable temperature gradients between rather closely spaced zones. The equipment must include the means of heating source and cooling substrates, complicated pumping stage, and facilities for gas admission, temperature and pressure monitoring, and technological process control. The high vacuum requirements make the process expensive and limit the substrate dimensions.
One more disadvantage of said known technology is limitation on the substrate materials: only substances retaining their physical, mechanical, optical, and their properties under the conditions of large pressure differences, high vacuum, and considerable temperature gradients can be employed. Besides, the requirement of matching between crystal lattices of the substrate and the growing film restricts the list of compounds suitable for deposition.
One of the major disadvantages of VPE is the strong influence of defects, present on the initial substrate surface, upon the structure of a deposited layer. The deposition of molecules from the vapor phase enhances/decorates defects on the substrate surface.
There is a method of film deposition from a solution. This method is limited to soluble compounds; most of solvents are highly hazardous liquids, which make manufacturing difficult and expensive. Also, the deposition process is hindered in cases of low wetting ability of the substrate surface.
Another method for thin crystal film manufacturing is described [see: U.S. Pat. Nos. 5,739,296 and 6,049,428 and in the following publications: P. Lazarev, et al., “X-ray Diffraction by Large Area Organic Crystalline Nano-films” Molecular Materials, 14(4), 303–311 (2001), and Y. Bobrov “Spectral properties of Thin Crystal Film Polarizers” Molecular Materials, 14(3), 191–203 (2001)], the disclosures of which are incorporated by reference in their entirety.
There are also known techniques for layer-by-layer electrostatic deposition of materials that form surface film alignments. One of the challenges of self-assembly techniques is the control of in-plane orientation of supramolecules. In bulk samples, uniform alignment is achieved by using lyotropic chromonic liquid crystal (LCLC) materials or by shear of polymer melts. One aspect of this method to provide a structure, wherein the film includes a polyion layer on the substrate, which may or may not be sheared, such that the polyion's charge is attracted to the charge of the substrate. Another aspect of the known method to provide a structure, wherein the film includes a LCLC layer disposed on the polyion layer, and wherein the LCLC layer may or may not be sheared. Another aspect of this invention to provide a structure, wherein of the LCLC layer material is attracted to the polyion layer's polarity. It is still another aspect of the discussed method to provide a structure, in which additional film layers of polyion and LCLC material may be added.
There has been no report on techniques for layer-by-layer electrostatic deposition of conjugated aromatic crystalline layers for obtaining a thin crystal film which is optically anisotropic and at least part of which is electrically conducting.
Intercalation [see: Woo-Chan Jung and Young-Duk Huh, Synthesis of Intercalation Compounds between a Layered Double Hydroxide and an Anionic Dye, Bull. Korean. Chem. Soc., 17, 547–550 (1996)] means mutual penetration (on the molecular level) of two or more substances from neighboring layers in a two or more layered structure. As a result, an intermediate compound is formed between layers of bulk materials. Cracks and pores in one layer can be filled by a material of the neighboring layer. This can lead to electrical and optical properties that were not inherent in the initial layered structure. In particular, intercalation can improve the optical and/or electrical contact between layers.
There has been no report on intercalation of conjugated aromatic crystalline layers for obtaining a thin crystal film which is optically anisotropic and at least part of which is electrically conducting.